Motion picture print film, the film that is shown in movie theaters, commonly employs an optical soundtrack along the edge of the film. A light source illuminates the optical soundtrack and a photosensor senses the light passing through and modulated by the soundtrack to produce an audio signal that is sent to amplifiers of the theater sound system. It is currently the practice to develop the sound track separately from the picture image on color motion picture film so that a silver image is left in the soundtrack area of the film, whereas all the silver is removed in the area of the picture, leaving only a dye image. There is current interest within the motion picture film processing community (because of growing laboratory processing costs and environmental concerns with the processing chemistry employed to process the soundtrack) to replace the silver soundtrack in motion picture print film with a "dye only" soundtrack. This would allow the entire width of film to be processed using the same chemistry, thereby realizing savings in processing and lessening environmental concerns.
Due to the spectral differences between the silver and the dye only soundtracks, the dye only sound track cannot be used in existing sound motion picture projectors. These sound systems incorporate a photodiode in the projector whose radiant sensitivity peaks at approximately 800-950 nanometers (depending on the type of photodiode) to detect the predominant infra-red (IR) radiation emitted by the tungsten lamp and modulated by the film's variable area silver soundtrack. The dye only sound track modulates light predominantly in the visible region of the spectrum. Although the photodiodes have some sensitivity in the visible range (approximately 380-760 nanometers) of the radiation spectrum, their lower sensitivity in this range, coupled with the lower emission of the light source in the visible range results in a very low input to the sound amplifier. The situation is further aggravated by the fact that the density range between the "clear" minimum density (D.sub.min) area and the "opaque" maximum density (D.sub.max) area is less for the dye only soundtrack. If the signal is too low for the amplification stage to operate properly (e.g. signal-to-noise loss), the sound quality will be degraded. Any change in existing projectors to accommodate dye only soundtracks must be easily reversed so that existing film with silver soundtracks can also be projected, due to the vast number of existing projection prints with silver soundtracks. Finally, the adaptation of existing projector sound systems to use of dye only soundtracks must not add significant cost due to the highly cost conscious and competitive nature of the motion picture industry.